Fracture Toughness (KJC), which represents the crack growth resistance of a material, is an important property of the material when evaluating the structural soundness of the material. However, conventional fracture toughness evaluation methods must require test-pieces having specified shapes and sizes in order to be reliable. In other words, a crack is preformed in a test-piece and, thereafter, stress and strain around the crack are mechanically analyzed, so that an appropriate test capable of determining the crack size and the crack growth direction can be conducted. Thereafter, fracture conditions can be determined using the test. Examples of representative fracture toughness tests are a compact tension test and a three-point-single edge notched bend test, each of which uses fatigue precracking. To execute the above-mentioned tests, a plurality of test-pieces having specified crack growth directions and crack sizes (refer to ASTM standards about the loading direction and crack growth direction) must be produced from a material. The crack growth of the material can be determined while fracturing the test-pieces.
However, the complex test procedure, which includes the fatigue precracking and the crack length evaluation, makes it difficult to evaluate the fracture toughness. Furthermore, the conventional fracture toughness evaluation method uses a destructive technique in which a test-piece is cut from a material. Thus, the conventional method cannot be adapted to actively operated industrial structures.
To mitigate the above-mentioned problems experienced in the conventional fracture toughness evaluation method, a variety of fracture toughness evaluation theories and models using indentation techniques have been studied and developed. However, the conventional theories and models to evaluate the fracture toughness of materials using the indentation techniques are limited to use in evaluating the fracture toughness of brittle materials. Furthermore, the conventional evaluation models using the indentation techniques are only effective when a subject brittle material is in a temperature range lower than the ductile-brittle transition temperature (DBTT) of the brittle material. If the brittle material is in a temperature range which is not lower than the ductile-brittle transition temperature (DBTT) of the brittle material, an indentation cannot form a crack in the brittle material, so that the fracture toughness evaluation of brittle materials using indentation techniques must be studied and developed.